When I took my car in for inspection in April, the dealership (I get a deal there for state inspections) gave me a list of things that were terribly, terribly wrong with it. Everything from brake lines to catalytic converter. One item was the battery. I've had the car for three years, which is the expected life of a battery in this climate...

Anyway, I had my usual shop verify 99.9% of the issues were not issues at all, but the battery WOULD need to be replaced soon.

So here's my question. If the alternator is more or less constantly engaged to keep charge on a battery that can't keep a charge, that would be a drag on fuel efficiency just like the AC compressor. Right? And it would effectively reduce the life of the alternator, to boot?

All I know is that with more than 60 days of 100+ degree heat here in Austin, my car is not having a fun time starting in the morning. Regardless of the responses here, I'm getting a new one ASAP.

 

I read somewhere where alternator drag (with the headlamps on, for example) would cost you ~1/2 MPG.


I know from experience that heavy A/C use costs me 2 - 3 MPG.




Heavy alternator use in hot weather could contribute to reduced alternator life, yes. This is more apt to happen with older cars where the voltage regulator for the electrical system was mounted on the alternator. I believe that Honda regulates voltage from the ECU now.

Hot weather such as you poor Texas folks are experiencing can truly be damaging to the battery! It is important to 'top up' the battery's cells with distilled water in summer. Unfortunately, on our '07, that requires removing the positive battery terminal to access the cell closest to that side. And THAT requires resetting the radio theft code and redoing the Idle Learn Procedure...

 

a dying battery cannot affect fuel economy. the problems with the cat converter and the other problems with your car does

 

Well I had a chat with an old friend of mine who is a retired master mechanic for BMW, and is now a full-time SCCA driver (driving a modified Austin Healy Sprite with a twin-tuebo Mazda b13 rotary), and he did confirm that a battery that can no longer accept full charge will indeed cause the alternator to remain engaged the entire time the car is running IF accessories are on, and that the drag from the alternator would result in a parasitic loss of power. The amount totally depends on the engine and the alternator. A torquey motor won't experience as much, etc. He figured based on his experience working on Hondas over the years that I could expect to see a loss of couple of MPG at most in regular driving.

So I dropped in a Civic (r51) battery yesterday.

 

I just wanted to conclude this thread with some numbers. The first tank after replacing the battery, according to Gas Cubby, was 27.9 MPG, marginally better than previously. I assume once the ECU got happy things started improving. The tank after that was 28.3 and yesterday after filling up I calculated 30.6, which is roughly back in line with last summer.

The gas was from the same Chevron station around the corner from my office, I filled up at roughly the same time (morning before work) each time, and pumped about the same quantity (between 9.26 and 9.56 gallons).

No other alterations to the car and the weather has been consistent... bloody hot. My driving patters and style were of course more or less the same. Can't think of any other variables.

So based on my admittedly quasi-scientific research, a bad battery CAN affect your fuel economy.

 

Interesting! Good conclusion... Makes sense... That's a wrap!

 

Chairman Kaga,

I see that you installed a Civic battery in your Fit. I compared the recommended (less desirable) 151R battery (Autocraft, 335 CCA at $106) to the 51R battery ((Autocraft, 500 CCA at $106). In sizes, the width is the same but the 51R is nearly 2 inches longer and almost 1 inch higher. How did you get it to fit?

Thanks and cheers.